Pigment particles dispersed in ink vehicle and printed on a surface of a substrate may have either high or low reflectance of incident light. This reflectance depends on several different factors. Inherent optical properties of the pigment particles significantly affect the reflectivity. Highly reflective “metallic” pigments are flake-shaped particles consisting primarily of either aluminum or other highly reflective metal. The metallic “effect” is achieved through the interaction of different individual characteristics, consisting primarily of particles size and shape as well as leafing or non-leafing behavior of the pigment.
Leafing and non-leafing properties are characteristics of the pigment determined by the pigment particles' ability to “float”. Leafing pigments float on a surface of printing ink film as a result of high interfacial tension. They form a coherent surface film whose reflective properties depend on the particles fineness. Non-leafing pigments are fully wetted by the binder and are distributed uniformly throughout the coating thickness. Leafing is described in Industrial Inorganic Pigments authored by G. Buxbaum; Wiley-VCH, ISBN 3-527-28878-3, 1998, p. 229. Since leafing and non-leafing pigments behave differently, they provide a very different appearance. For example, particles of leafing aluminum pigments will distribute themselves more evenly across the surface of the film, they will provide a chrome-like finish.
A common method manufacturing “leafing” pigment particles is by ball milling of the pigment in the presence of various lubricants as is described in U.S. Pat. No. 6,379,804 in the names of Ackerman et al. Stearic acid is usually used for this purpose. The acid repels a liquid ink or paint vehicle from the surface of pigment pushing particles in the direction of interface between air and the ink surface. As a result, the platelets float to the surface of the print exhibiting a mirror-like appearance. However, a disadvantage of this method is a reduced abrasive resistance: the prints containing such “leafing” particles have poor resistance to its abrasion because the acid prevents adhesion of the ink vehicle to the surface of the flake.
Images printed with non-leafing particles, in contrast to the “leafing” pigment do not have a smooth mirror-like appearance. Instead, they have a speckling “sparkle-like” finish. On the other hand, these printed images have very good abrasive resistance because an ink vehicle forms a strong bond with a non-modified surface of pigment particles. It would be advantageous to fabricate a highly reflective image printed with non-leafing pigment particles distributed in the layer of ink parallel to the surface of the ink similar to leafing particles.
It is possible to orient particles in the layer of ink by an external action. For instance, U.S. Pat. No. 2,418,479 in the name of Pratt et al., references fabrication of bright metal paint films by brushing or knifing.
Pratt in U.S. Pat. No. 2,418,479 also discloses a method of making particles with leafing-like properties by their alignment in applied magnetic field. Particles used for this application have to be magnetic and preferably reflective. Particles being dispersed in liquid ink, coated on a surface of a substrate and exposed to an external magnetic field, tend to align with their easy axis along lines of the field. The particles' positions become fixed after their orientation by curing of the binder. In materials science, the term “easy axis” is referred to the energetically favorable direction of the spontaneous magnetization in a ferromagnetic material. This axis is determined by various factors, including the magnetocrystalline anisotropy and the shape anisotropy. The two opposite directions along the easy axis are usually equivalent, and the actual direction of the magnetization can be either of them.
Alignment of magnetic particles, dispersed in a layer of an organic binder on a surface of a substrate and exposed to an external magnetic field, is described in many books and patents, for example, Magnetic Recording by C. Denis Mee; McGraw-Hill Book Company, ISBN 0-07-041271-5, Volume 1, p. 164, or The Complete Handbook of Magnetic Recording, by Finn Jorgensen; TAB Professional and Reference Books, ISBN 0-8306-1979-8, 1988.
C. D. Mee describes the alignment of particles in a recording media by applying a magnetic field parallel to the web transport direction. The preferred configuration of the magnetic pole pieces of a permanent magnet circuit was one of opposite poles on both sides of the web, which have no perpendicular field component in the center plane. The author also described various magnetic devices for orientation of magnetic particles along the web direction. However, there is a particular common feature in alignment of particles by these devices relative to the surface of the substrate. The references clearly state that for recording tapes the particles are aligned with their easy axis along direction of the web.
Most of magnetic materials used in recording media are quasi one-dimensional having one dimension much larger than two others, such as a shape similar to wires or needles. As is shown in FIG. 1a, a particle shaped as a platelet 101 can be considered as a two-dimensional (XY) physical body due to the large aspect ratio of its dimensions: X and Y which are substantially larger than Z. When a platelet is induced by a magnetic field, it experiences a magnetic torque whenever its dipole vector is not parallel with the external field. The existence of the magnetic torque results in the rotation of the particle with its easy axis along direction of magnetic lines 102 of an external field; and the dipole vector of the platelet becomes parallel to the magnetic field vector. The particle does not rotate and becomes stabilized in this position until the field is removed. In other words, the particle becomes oriented with its longest diagonal, which can be considered as a major dimension, for example the coordinate X, parallel with lines of applied magnetic field, as is shown FIG. 1a. 
However, the second dimension, Y, may not be always parallel to the substrate. In fact we have noted that it is always tilted at some small angle to direction of applied magnetic field. This small tilt is not very important for the overall performance of recording media, however, it becomes very significant in the paint and printing industries for the production of durable highly reflective coatings on various substrates.
For many decades, attempts have been made to fabricate bright coatings with reflective flake material. Pratt et al., in 1947 publicly disclosed in U.S. Pat. No. 2,418,479 a process of orienting metallic flake pigments for making bright coatings. Pigments, such as ferromagnetic flakes, in paint films are positioned on a simple, planar surface by reaction to a magnetic field. Both the article surface and the flakes are located in the direction of the magnetic field. This method requires that the article surface on which the film is disposed lie between magnetic poles so that each long or major dimension of the particles will align itself along the magnetic field direction, as does a needle of a compass. Pratt et al., disclose a method which includes placing of a wet film containing ferromagnetic flakes to the action of a magnetic field and to directional angle between the film and the field being made to vary from parallel to the perpendicular at short intervals of time until the film has dried. Directional changes between the film and the field can be produced either by rotation of the film or by planar rotation of magnetic field. Electromagnetic system described in the patent generates a rotating magnetic field allowing the flake to change its direction with the frequency from 5 to 10 Hz. The magnetic field changes its direction by 90°. The magnets in one of the sets were hollow and the substrate was passing continuously at a predetermined rate through the center on these hollow magnets.
Although Pratt's invention disclosed in U.S. Pat. No. 2,418,479 has made an advance in the art it has some disadvantages. The use of Pratt's method is impractical for large surfaces of articles since the magnetic field strength would have to be extremely large and difficult and costly to construct. In addition, such method as described would not operate to orient a majority of the flakes disposed in a film or coating on a curved or complex shaped, non-planar article surface. One example of such a surface is an annular or airfoil shaped component of power generating apparatus such as a gas turbine engine.
James Peng in U.S. Pat. No. 4,859,495, assigned to Eastman Kodak, and issued 1989 discloses a method of making a magnetic recording film having the magnetic particles oriented in any given direction which comprises applying a magnetic paint to a substrate, subjecting the substrate containing the magnetic paint in an unfixed condition to a rotating magnetic field having magnetic components that lie only in a plane perpendicular to the given direction and fixing the magnetic paint. Orientation of the flakes occurs in layer of organic binder containing dispersed magnetic flakes coated on a surface of moving web. One magnetic system describes four Helmholtz coils. The second embodiment describes a system for a rotating magnetic field having no Z component while the magnetic paint layer is moving at a rapid rate on the substrate in a fluid condition. This embodiment is suitable for making magnetic media in very wide widths for example up to thirty and even fifty inches in width because it is not limited in the same manner as that of the Helmholtz coils. The embodiment utilizes two conductive plates such as, for example, copper plates which are disposed above and below the moving web. The current in the plate on the top of the moving web traverses across the width of the moving web. The plate beneath the moving web has a current which traverses the plate in the direction of motion of the moving web due to the voltage source. The two currents impressed by two voltage sources 90° out of phase with respect to each other and this causes a rotating magnetic field having components only in the X and Y direction with no magnetic component out of the plane of the web or in the Z direction. Disadvantages of this method are also lack of its usefulness for large surface substrates, problems with uniformity of orientation over the large surface, destruction of a proper orientation by the exit magnetic field and lack of practicability of in-field UV curing.
United States patent application US 2004/0052976 in the names of Buczek et al. discloses alignment of non-spherical particles with their major dimension oriented generally along an article surface in respect to which the particle is disposed. The particles, disposed in a fluid medium, the viscosity of which can be increased to secure the particles' in position, are positioned using a force on the particles. The force includes a torque force from a magnetic field, force from flow of the fluid medium, the force of gravity, and the force of surface tension alone or in combination with the force of gravity. For control of brightness of or reflection from a surface, coatings and sheets have used non-spherical metallic particles in the shape of flakes having a major dimension, with the relative orientation of the flake and the major dimensions in respect to the article surface determining the degree of brightness or reflection. Although this method has some usefulness for making of bright paint coatings it lacks practicality for printing bright images on the top of a wide web moving for example at speeds of 100-500 ft/min.
Another United States Patent which relates to magnetic alignment of particles is U.S. Pat. No. 5,630,877 in the names of Kashiwagi et al. who disclose alignment of magnetic particles or flakes dispersed in organic binder and exposed to an external magnetic field. A method and an apparatus are taught for producing a product having a magnetically formed pattern, capable of forming a desired pattern in diversely different shapes with a clear visual recognizability, at high speeds using a simple procedure, and a painted product produced by these methods and apparatus. However, the patent does not describe how to make bright coatings with X and Y components parallel to the surface of a wide web moving high a speeds.